1. Field of the Invention
This invention relates to a silicon single crystal pulling-up apparatus.
2. Description of the Related Art
A single crystal of silicon used as the material of a substrate for a semiconductor device is produced mainly by Czochralski (CZ) process which produces single crystal ingot of silicon in principle by supporting a crucible rotatably within a chamber, melting a silicon material within the crucible, dipping a seed crystal hung rotatably from above into the surface of the melted silicon, and pulling-up the seed crystal.
A number of very precise temperature controls with respect to a pulling-up of the silicon single crystal are requested recently. For example, they are a control of heat history received by a pulled up crystal, a delicate thermal control on a solid liquid interface and a total temperature control of liquid and so on. Uniformity of concentration of oxygen and resistivity in the crystal can be obtained by the above mentioned controls.
The crucible support of the conventional silicon single crystal pulling-up apparatus comprises a rotational shaft extending into the chamber from a lower opening in the chamber and a crucible-shaped protector of isotropic carbon (so called carbon crucible) fixed at an upper end of the rotational shaft. A crucible of silica glass is accommodated within the protector. The carbon crucible is thick enough to prevent thermal deformation of the crucible.
The conventional carbon crucible covers the entire silica glass crucible with the same material so that both the coefficients of thermal expansion of those materials are required to be similar to each other. Therefore, only isotropic carbon having a particular physical property can be used; namely, the kinds of selectable material are limited. The conventional carbon crucible is produced by drilling an isotropic carbon block, so the producing cost is high. In addition, such isotropic carbon block discharge a large amount of occluded gas from its pores to pollute the silicon single crystal and adversely affect the physical properties of the silicon single crystal.
The carbon crucible has means for preventing its breakage, for example, preventing a stress concentration by dividing its body into, for example, three parts. However, its service life is not necessary long, the crucible must be exchanged after about twenty times of use and therefore the running cost is expensive.
The silica glass crucible is covered with the thick carbon crucible, so that its thermal response to a change in the output of the heater is low, and it is difficult to rapidly control the temperature of the melted silicon. This problem has come more noticeable as the diameter of the crucible becomes larger.